1. Field of the Invention
The present invention relates to a hydraulic braking pressure control apparatus for vehiclesy, and more particularly to a hydraulic braking pressure control apparatus for vehicles having an input hydraulic chamber communicating with an output port of a master cylinder, and an output hydraulic chamber communicating with a wheel brake and being adapted to generate a hydraulic braking pressure in accordance with a hydraulic pressure in the input hydraulic chamber, said apparatus having a hydraulic braking pressure modulator, wherein the volume of the output hydraulic chamber can be increased in accordance with the supply of a hydraulic control pressure to a control chamber when a wheel is about to become locked.
2. Description of the Prior Art
In a conventional hydraulic braking pressure control apparatus for vehicles, a piston is operated in response to the variation of a hydraulic pressure within an input hydraulic chamber so as to reduce the volume of an output hydraulic chamber and thereby generate a hydraulic braking pressure, in accordance with the pressure in the input hydraulic chamber, from the output hydraulic chamber. During an anti-lock operation, the piston is displaced in a direction opposite to that in the abovementioned case by means of a control liquid pressure supplied to a control chamber, to increase the volume of the output hydraulic chamber.
In the above conventional hydraulic braking pressure control apparatus, a hydraulic braking system is divided into two parts, one of them extending from the master cylinder to the input hydraulic chamber, and the other extending from the output hydraulic chamber to a wheel brake. Accordingly, when the hydraulic braking system is filled with hydraulic fluid, such as oil, it is required that those two parts be charged with the oil separately. Moreover, since the piston is in operation at all times during a braking operation, the number of its operation strokes is great. This may lead to deterioration of the durability of the control apparatus.
The assignee of the present invention has already proposed, for example, in Japanese Patent Application Laid-Open No. 234061/1985 Specification, a hydraulic control apparatus provided with a valve mechanism of a normally opening type in a partition located between input and output hydraulic chambers, which valve mechanism is adapted to disconnect the input and output hydraulic chambers from each other during an anti-lock operation, in order to integrate the hydraulic braking system into a single line extending from a master cylinder to a wheel brake, thereby facilitating a working oil charging operation, and to reduce the number of strokes of the piston to improve the durability of the control apparatus.
According to such a hydraulic control apparatus, the above-mentioned problems can be solved. However, when the vehicle runs on a bad road, or when anti-lock control is made excessively due to the trouble of an antilock control means, an abnormal increase of the hydraulic pressure in a control chamber causes the volume of the output hydraulic chamber to increase abnormally.
For this reason, a negative pressure may be developed in the hydraulic system between the output hydraulic chamber and the wheel brake to cause the generation of trapped air bubbles.